Multidrug resistance (MDR) is a common problem in various types of cancer. One important factor in the development of MDR is overexpression of P-glycoprotein, encoded by the MDR1 gene. Morphine is the opioid of choice for moderate to severe cancer pain, and is a substrate of P-glycoprotein. Recently, morphine has been shown to induce P-glycoprotein expression in the rat brain. Using Western blot analysis and cytotoxicity assays respectively, we have investigated the effects of short-term (72 h) morphine treatment on P-glycoprotein expression in a panel of human cancer cell lines, and its effects on cellular resistance to the known P-glycoprotein substrates, vinblastine and colchicine. The effect of morphine on P-glycoprotein expression and activity in the mouse fibroblast NIH-3T3 cell was assessed to establish whether morphine effects are species specific. Short-term exposure to morphine did not result in any significant differences in P-glycoprotein expression or activity in any cancer cell lines. Morphine pre-treatment resulted in a moderate but significant increase in sensitivity of NIH-3T3 cells to vinblastine, but not colchicine. This study suggests that morphine effects may be cell-type specific. Importantly, however, it appears that short-term morphine treatment does not affect the MDR phenotype of tumour cells.

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